Euradwaste '08 - EU Bookshop - Europa

proved, with amongst others the determination of activity coefficients for Eu in powellite. The fact
that Am, Pu, Cm can be structurally incorporated into the host minerals is a universal observation.
If one succeeds in developing further thermodynamic data for the aqueous – solid solution equilibria,
these data will be relevant to every system.
Validation of key mechanisms of glass dissolution in integrated near-field conditions
The dissolution of glass in near-field conditions is believed to be driven by a set of key mechanisms,
i.e. the transport of water into the glass, followed by ion exchange, and transport of silica out
of the glass. The transport of silica out of the glass is influenced much by the near-field, by means
of diffusion into and sorption on overpack corrosion products and backfill clay, and by advection or
precipitation in the near-field. Prior to NF-PRO, this model, and the parameter values involved,
have been developed based on tests of the various simplified subsystems (e.g. glass/clay water, clay
water/magnetite etc.). The objective of the integrated glass dissolution experiments in NF-PRO was
to validate this glass model and the underlying key mechanisms in realistic near-field conditions,
where all processes are coupled in a realistic set-up. The results were used to support geochemical
models, which were then applied for long term predictions. Tests were performed with glasses
SON68 and the blended Magnox-UO2 glass. The dissolution of these glasses was followed in reactors
where the glass was in contact with (1) a layer of compacted Volclay KWK, or (2) a layer of
magnetite powder, followed by a layer of Volclay. These tests allowed seeing the impact of direct
contact with the clay, and the impact of the presence of a layer of magnetite between the glass and
the clay. Parallel tests were performed with addition of amorphous silica to the Volclay or magnetite.
These tests allowed assessing the effect of saturation of the Si sorption sites, which simulates
the long term. Although the reference long term in situ temperature is 50°C or less, the tests were
performed at 90°C, to accelerate the processes.
The fundamental understanding of the glass dissolution in aqueous solutions was confirmed:
(1) Initial fast dissolution of the glass matrix
(2) accumulation of dissolved glass constituents in solution
(3) accumulation of less soluble glass constituents on the glass surface by forming a surface
layer of solid reaction products (including formation of a so called “gel layer”)
(4) slow-down of reaction rates due to the accumulation of dissolved silica in the aqueous
phase adjacent to the dissolving glass or within the pore water of the gel
(5) approach of a residual reaction rate which can be up to 10000 times lower than the initial
rate
(6) retardation of process (5) by adsorption of dissolved silica on iron corrosion products .
The quantitative reproduction of the data with the geochemical codes was nevertheless not always
satisfying. The rate increasing effect of magnetite can be simulated, but a better description of the
combination of geometric, hydrodynamic, thermodynamic and kinetic constraints is necessary.
The silica/clay system must be better understood (kinetics, solubility, sorption, temperature
effect).
The hypothesis of instant sorption equilibrium on the magnetite must be revised.
In addition to silica sorption, silica precipitation on the magnetite may take place.
The pH evolution at the glass interface must be better understood.
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